1. Field of the Invention
The present invention relates to an optical scanning apparatus including a plurality of light sources that can emit a laser beam based on image data so that an image can be formed on an image carrier with the laser beam.
2. Description of the Related Art
An electrophotographic type image forming apparatus including a laser element capable of functioning as a latent image forming exposure unit is required to have higher capabilities, for example, in resolution and processing speed.
The image forming apparatus drives (on/off controls) a laser element based on image data. In general, the resolution of an image can be improved if the on/off control speed (hereinafter, referred to as “modulation speed”) increases although the amount of increase in the modulation speed may be limited.
An image forming apparatus with a laser element emitting a single laser beam is required to excessively increase the modulation speed to attain a requested resolution in a main scanning direction (i.e., the longitudinal direction of an image carrier on which a latent image is formed) as well as in a sub scanning direction (i.e., a rotational direction of the image carrier).
Accordingly, instead of increasing the modulation speed, it is useful to increase the number of laser beams, i.e., the number of light emission points (i.e., light sources), to improve the resolution in the sub scanning direction. For example, if the number of laser beams is increased from 1 to 4, the resolution in the main scanning direction and the sub scanning direction can be doubled at the same modulation speed as the single beam.
The semiconductor lasers (i.e., laser elements) can be classified into two types depending on the direction of light emission. More specifically, an “edge-emitting laser” emits a laser beam in a direction parallel to an active layer. A “surface-emitting laser” or a “vertical-cavity surface-emitting laser (VCSEL)” emits a laser beam in a direction perpendicular to an active layer. In general, an edge-emitting laser is used for an electrophotographic type image forming apparatus.
However, the edge-emitting laser has a technical drawback in that the number of laser beams cannot be increased so much. On the other hand, the surface-emitting laser has a structure easy to increase the number of laser beams because a relatively large number of light sources can be arrayed on a surface.
Therefore, an electrophotographic type image forming apparatus, if it employs a surface-emitting laser capable of simultaneously emitting numerous laser beams, can realize high resolution and high speed processing.
The light quantity of a laser beam emitted from a semiconductor laser (i.e., laser diode (LD)) varies depending on the temperature. More specifically, the temperature of a laser diode increases when light emission from the laser diode continues for a long time. As a result, the light quantity of a laser beam on a photosensitive drum surface (i.e., image carrier) cannot be maintained at a constant level. An image formed on the photosensitive drum includes unevenness or fogging.
To solve this problem, as discussed in Japanese Patent Application Laid-open No. 2002-335038, a conventional light-emitting element driving apparatus depending on a combination of voltage drive and current drive can correct a variation in laser light quantity that occurs due to a temperature rise in a laser diode.
More specifically, the light-emitting element driving apparatus detects a reduction in a terminal voltage of a laser diode due to heat generation in the laser diode, as a temperature rise, in a constant current drive operation. The light-emitting element driving apparatus controls a drive voltage based on a detection value so as to correct a variation in the laser light quantity.
Furthermore, as discussed in Japanese Patent Application Laid-open No. 11-291547, a conventional image forming apparatus corrects droop characteristics which vary depending on image data. According to this image forming apparatus, when light emission of a laser diode is based on a current drive, a drive current corresponding to image data is added to a drive current for an auto power control (APC), i.e., automatic light quantity control. A detailed laser diode driving operation is described below with reference to FIG. 11 and FIGS. 12A through 12C.
FIG. 11 illustrates a laser drive control circuit included in the conventional image forming apparatus. The laser drive control circuit illustrated in FIG. 11 includes a laser diode (LD) 101 and a photo diode (PD) sensor 102 which cooperatively form a laser chip. Furthermore, the laser drive control circuit includes a bias current source 103 for the LD 101 and a pulse current source 104 for the LD 101.
A modulation unit 105 receives an image signal “DATA” and outputs a pixel modulation signal to a switch 106. The switch 106 performs an on/off control based on the pixel modulation signal supplied from the modulation unit 105. Namely, the switch 106 controls the current supplied to the LD 101 from the pulse current source 104. Thus, the LD 101 emits a laser beam based on the pixel modulation signal.
The PD sensor 102 monitors (detects) light emission from the LD 101 and generates an output signal (i.e., detection result) which is sent to a current/voltage (I/V) converter 107. The current/voltage converter 107 converts the output signal of the PD sensor 102 into a voltage signal. An amplifier (Gain) 108 receives the voltage signal from the current/voltage converter 107 and amplifies the received voltage signal. An APC circuit 109 receives the amplified voltage signal from the amplifier 108 and performs a feedback control of an output current amount supplied to the bias current source 103. In other words, the APC circuit 109 equalizes a bias light quantity of the LD 101 with a desired light quantity.
The LD 101 has droop characteristics which depend on an on-period of the LD 101. Even if the drive current amount is controlled to be constant during a scanning operation, the light emission quantity of the LD 101 gradually decreases due to the droop characteristics. Similar phenomenon appears when the drive current control is performed based on a pulse signal. A laser diode having strong droop characteristics tends to cause a large attenuation in the light emission quantity when the on-period of the laser diode is long.
A current control unit 110 outputs a correction signal to a pulse correction current source 111. The correction signal is dependent on the on-period of the LD 101 in a scanning operation. The pulse correction current source 111 supplies current based on the correction signal. Thus, a sum of current values output from the pulse current source 104 and the pulse correction current source 111 is supplied to the LD 101 as a pulse drive current during a scanning operation. Furthermore, a sequence controller 112 is in communication with the APC 109 and the current control unit 110.
FIGS. 12A through 12C illustrate an exemplary change in the light emission quantity of the LD 101 during an on-period, according to the laser drive control circuit which includes the current control unit 110 and the pulse correction current source 111. FIG. 12A illustrates image data being set for light emission from the LD 101. FIG. 12B illustrates drive current flowing across the LD 101. FIG. 12C illustrates the quantity of light emitted from the LD 101.
In FIGS. 12B and 12C, solid lines represent the data measured when the laser drive control circuit includes the current control unit 110 and the pulse correction current source 111. Dotted lines represent the data measured when the laser drive control circuit does not include the current control unit 110 and the pulse correction current source 111.
Namely, if the drive current supplied to the LD 101 is constant as indicated by a dotted line in FIG. 12B, the light emission quantity of the LD 101 during an on-period gradually decreases due to the above-described droop characteristics as indicated by a dotted line in FIG. 12C.
On the other hand, as indicated by a solid line in FIG. 12C, the light emission quantity of the LD 101 during an on-period can be maintained at a constant level when the laser drive control circuit includes the current control unit 110 and the pulse correction current source 111.
As described above, an electrophotographic type image forming apparatus can use a surface-emitting laser capable of emitting numerous laser beams to realize high resolution and high speed processing.
However, the surface-emitting laser includes a plurality of laser diodes (light emission points) disposed densely on the same chip. The self heat generation from one laser diode has significant effects on neighboring laser diodes disposed on the same chip. Therefore, if the conventional light quantity correction is applied to a surface-emitting laser without considering the effects given from the neighboring laser diodes, a target laser diode (i.e., a control object) cannot generate a laser beam whose light quantity is stable during an on-period.